Soak beans overnight in 8 cups water. (Alternatively, place beans and 8 cups water in a large pot. Boil for 2 minutes. Remove from heat and let stand for 1 hour.)
Drain the beans and discard the soaking liquid.
Combine oil, mustard seeds, chili powder, cumin and cardamom in a saute pan and stir for about 30 seconds.
Add the onions, mushrooms, tomatillos, and water. Cover and cook, stirring occasionally, for about 5 minutes.
Uncover and stir often, cooking for about 10 to 15 minutes until the vegetables are nicely browned.
Add the broth and tomato paste and mix well.
Place the beans in a slow cooker.
Pour the hot vegetable mix over the beans.
Turn the heat to high, place on the lid, and cook until the beans are creamy. This should take about 5 hours.
Optional to garnish it with your version of sour cream, cheese, and/or avocado.
